The invention relates to a flat panel display and in particular to a top emission type electro-luminescent (EL) device and method for fabricating the same.
Electro-luminescent (EL) devices, such as organic light-emitting diodes (OLEDs), are active lighting devices using organic materials. The EL device typically comprises an anode, a cathode, and an EL medium layer disposed therebetween. When an electrical potential difference is applied between the anode and the cathode, electrons and holes are injected into the EL-medium layer from the cathode and the anode, respectively. The injected electrons and holes are then recombined, releasing energy as light.
FIG. 1 illustrates a conventional bottom emission type electro-luminescent device. As shown in FIG. 1, a buffer layer 102 is formed on a substrate 100. A thin film transistor 111 is disposed on the buffer layer 102, comprising an active layer 104, a gate dielectric layer 106, a gate electrode 107, and source and drain electrodes 109. The active layer 104, such as a polysilicon layer, is. disposed on the buffer layer 102, comprising source and drain doping regions 105. The active layer 104 is covered by an insulating layer 106, such as a silicon nitride layer, serving as the gate dielectric layer. The gate electrode 107 is disposed on the gate dielectric layer 106 overlying the active layer 104 and covered by an interlayer dielectric (ILD) layer 108. The source and drain electrodes 109 are respectively disposed on both sides of the gate electrode 107 and electrically connected to the source and drain doping regions 105, respectively, by the contact holes in the ILD layer 108 and the underlying insulating layer 106. A first passivation layer 110 covers the thin film transistor 111 and the ILD layer 108, comprising a via hole to expose the source/drain electrode 109. A transparent electrode 112, such as an indium tin oxide (ITO) layer, is formed on a portion of the first passivation layer 110 and electrically connected to the exposed source/drain electrode 109 through the via hole. A second passivation layer is disposed on the first passivation layer 110 overlying the thin film transistor 111. An EL medium layer 116 covers the second passivation layer 114 and the transparent electrode 112. An opaque electrode 118, such as a metal material, is formed on the EL medium layer 116.
In FIG. 1, the transparent and opaque electrodes 112 and 118 serve as the anode and the cathode, respectively. Thus, the light is emitted in a downward direction from the EL medium layer 116 through the transparent electrode 112, providing a bottom emission type EL display device. The light-emitting area of the bottom emission type EL display device, however, is limited by thin film transistors. For example, the aperture ratio of the EL device is reduced while the number of the thin film transistors is increased. Accordingly, power consumption must be increased to maintain the EL display panel brightness, reducing the lifetime of devices.